Add support for the `:paste` command in the REPL

[Gedochao]

The :paste command was (and is) supported by the Scala 2 REPL, while it is visibly absent in Scala 3.

scala -S 2.13
Welcome to Scala 2.13.15 (OpenJDK 64-Bit Server VM, Java 17).
Type in expressions for evaluation. Or try :help.
scala> :help
All commands can be abbreviated, e.g., :he instead of :help.
(...)
:paste [-raw] [path]     enter paste mode or paste a file
(...)
scala> :paste
// Entering paste mode (ctrl-D to finish)

println("I")
println("paste")
println("stuff")

// Exiting paste mode... now interpreting.
I
paste
stuff

scala> 

[SethTisue]

:paste was originally added to Scala 2 at a time when JLine didn't yet have support for multiline editing

I doubt it's worth re-implementing in Scala 3. but, see discussion at https://users.scala-lang.org/t/solved-cannot-use-paste-in-scala3-repl/7587/19

[Gedochao]

Similarly to :sh, as per #21657 (comment), calling :paste should print a warning about it not being supported in the Scala 3 REPL, and about it not being needed, since JLine supports multiline editing.

@SethTisue should we deprecate :paste in Scala 2?

• some Scala 2 REPL commands should be "deprecated" if not implemented in Scala 3 bug#13080

[SethTisue]

@SethTisue yes; I've added a note to the Scala 2 ticket